Investigation on the low frequency dielectric response of ground-wall insulation of rotating machine windings

In order to assess the quality of stator winding insulation, laboratory and field instruments measuring the charge and discharge currents flowing through rotating machine ground-wall winding insulation were developed. These instruments apply a dc voltage step during a variable charging time at various voltage levels followed by a discharge step during which the insulating dielectric is shorted to ground. The current is continuously monitored during both the charge and the discharge steps. An investigation was conducted for epoxy-mica insulation systems in laboratory as well as on complete windings in the field.